Barnes et al in U.S. Pat. Nos. 2,975,603; 3,086,370; and 3,217,503 disclose processes for preparing ice products containing from about 25 to about 120 milliliters of carbon dioxide, or other suitable conditionally-stable-hydrate-forming gas, per gram of frozen product. According to one aspect of these related disclosures, carbonated ice was prepared by subjecting aqueous liquid to a carbon dioxide pressure of at least about 200 psig and preferably less than 600 psig; maintaining the aqueous liquid and the carbon dioxide in contact for a time sufficient to permit absorption in the liquid of carbon dioxide in bound form; and freezing the reaction mixture which contained carbon dioxide hydrate crystals suspended within unreacted aqueous liquid. The products resulting from the processes of these patents were less uniform than desired, and the processes themselves were difficult to control to achieve consistent results.
Adler et al, in U.S. Pat. No. 3,220,204, state that while the prior art procedures of Barnes et al produced products which would retain a high level of carbonation during frozen storage, the products had a tendency to explode or pop during dissolution of the product to release the gas. Adler et al indicate that when the Barnes et al carbonated ice products were added to water or milk, they would frequently explode in the glass. To correct this, Adler et al subjected a thin film of water to carbon dioxide gas at a pressure and temperature above the eutectic point of the water, the temperature being low enough to form a hydrate. They stated that, as a practical matter, in order to operate under controllable conditions, hydrate should be produced at a pressure above 200 psig and at a temperature above 0.degree. C., in order to maximize hydrate formation while minimizing collateral formation of water ice. After suitable hydrate formation, the reaction mixture containing water and hydrate crystals was frozen at a temperature below -3.degree. C. This process did improve process control, but gas hydrate levels were not as high as desired and the products typically exhibited great variations in gas hydrate concentration.
In U.S. Pat. No. 3,255,600 to Mitchell et al, there is disclosed a process for forming carbonated ice wherein liquid carbon dioxide and liquid water or water ice are mixed under controlled conditions. The patentees indicate that they discovered that liquid carbon dioxide results in a more rapid formation of the product while permitting more accurate control of the operating conditions. It has been our experience, however, that the use of liquid carbon dioxide requires the use of great quantities of energy and produces a product which loses significant gas content before it can be commercially distributed; and it has the popping and cracking problems associated with the earlier prior art.
Throughout this evolution of gasified ice products involving reactions above the freezing point of water, Mitchell et al disclose in U.S. Pat. No. 3,333,969, that the problem of uneven release of the gas had persisted. Mitchell et al focused on a method for subdividing carbonated ice into discrete particles while maintaining the temperature of the ice below 0.degree. C., and then compacting the discrete particles to form them into an adhered mass or briquette to eliminate the explosive release of carbon dioxide during carbonation. This process actually resulted in a decrease in final gas content.
It is apparent from the foregoing discussion of the prior art that the problems associated with achieving gasified ice products having high gas contents, good storage stability, and uniform gas release have been significant concerns. These problems were further compounded by the unpredictability inherent in these processes which relied upon the spontaneous initiation of nucleation within the aqueous liquid. Therefore, there remains a present need for improvements in the preparation of gasified ice products which will permit more precise process control and enable the production of more uniform gasified ice products.